Hydrokinetic coupling apparatus, in particular for motor vehicle, comprising a needle thrust bearing between the piston and the turbine hub

ABSTRACT

The invention concerns a hydrokinetic apparatus, in particular for motor vehicle, comprising a casing ( 10 ); a turbine wheel ( 16 ) secured in rotation to a hub ( 30 ); a lock-clutch comprising a piston ( 50 ) whereof the radially inner part ( 54 ) is mounted sliding along an outer cylindrical shaft ( 58 ) belonging to the central ring called piston ( 50, 54 ) whereof a rear end surface ( 64 ) extends opposite a transverse surface belonging to the turbine wheel ( 16 )/hub ( 30, 86 ) assembly, and comprising stop means ( 84 ) axially interposed between the central ring and the turbine wheel/hub assembly. The invention is characterized in that are provided radial centering means for the axial stop ( 84 ) relative to the hub ( 30, 5 ) or relative to the piston ( 50, 54 ) or relative to the web ( 104 ).

[0001] The present invention relates to hydrokinetic coupling apparatus,especially for a motor vehicle.

[0002] More particularly, the invention relates to a hydrokineticcoupling apparatus of the type comprising a casing having a generallytransversely oriented wall adapted to be coupled in rotation to adriving shaft, a turbine wheel including an internal plate element andcoupled in rotation to a hub which is adapted to be coupled in rotationto a driven shaft, a lock-up clutch interposed between the turbine wheeland the transverse wall, and comprising, in particular, arranged axiallybetween the turbine wheel and the transverse wall, a generallytransversely oriented piston, the radially inner portion of which ismounted for sliding movement along a cylindrical outer surface of acentral sleeve, which is referred to as a guide sleeve for the piston,and an annular, transverse rear end face of which lies facing a fronttransverse face of the assembly consisting of the turbine wheel and hub.

[0003] One example of a hydrokinetic coupling apparatus of this kind isdescribed and shown in the document FR-A-2 634 849 or U.S. Pat. No.4,926,988.

[0004] In this type of design it is firstly necessary to provide anabutment for limiting the axial displacements of the piston towards theturbine wheel, and secondly to provide an abutment element acting as aspacer, which is interposed between the central sleeve and the hub ofthe turbine wheel, so as to determine a precise axial position betweenthese two elements, and also in order to reduce to a minimum frictioneffects between these two elements, one of which rotates with respect tothe other.

[0005] In the design according to the state of the art as constituted bythe documents cited above, the piston abutment consists of an annularring which is fitted in a groove formed in the central sleeve, while thespacer abutment between the central sleeve and the turbine wheel hub isan additional component which is made for example in the form of awasher.

[0006] This design is accordingly complex, in that it includes twocomponents and necessitates, for the abutment for the piston, anadditional machining operation in the central sleeve.

[0007] In order to overcome these drawbacks, the document FR-A-2 782 362proposed thrust bearing means which are interposed axially between theannular transverse rear end face of the central sleeve and the saidfront transverse face of the assembly consisting of the turbine wheeland hub, whereby to limit friction between the said two elements thatare rotatable with respect to each other, a portion of the thrustbearing being disposed facing a rear transverse face of the radiallyinner portion of the piston, so as to constitute an axial abutmentdefining the maximum rearward position of the piston when the clutch isdisengaged.

[0008] In the design proposed in that document, the abutment means aremade in the form of a single one-piece component which is interposedaxially between the central sleeve and the hub of the turbine wheel, andthe radial size of this component is such that it performs both of theabutment functions mentioned above at the same time.

[0009] This single abutment component is preferably made in a materialhaving optimum friction characteristics so as to reduce friction betweenthe rotating parts.

[0010] However, this design is not entirely satisfactory since theabutment is not positioned, especially in the radial direction. Inaddition, the compromise in the selection of a material having goodenough ability to reduce friction, while being strong enough not to wearrapidly, is hard to optimise.

[0011] In order to provide a remedy for this disadvantage, the inventionproposes a hydrokinetic coupling apparatus of the type mentioned above,characterised in that radial centring means are provided for centringthe axial abutment with respect to the hub or with respect to the pistonor with respect to the plate element.

[0012] Thanks to the invention, the axial abutment is centred in theradial direction. Preferably, the abutment means consist of an axialbearing with rolling elements, which is interposed axially between theannular transverse rear end face of the central sleeve and the saidtransverse front face of the assembly consisting of the turbine wheeland hub, and in that [sic] at least one ring of the axial thrust bearingincludes a radially oriented annular portion which lies facing the saidtransverse rear face of the radially inner portion of the piston.

[0013] Because of this design, abutment means are provided which havereduced friction and high reliability, the design of which, since itmakes use of one or two rings between which the rolling elements arearranged, offers numerous possibilities for the arrangement of thisaxial thrust bearing with rolling elements between the variouscomponents.

[0014] The axial thrust bearing is preferably a needle bearing.

[0015] According to further features of the invention:

[0016] the axial thrust bearing has at least one front ring interposedaxially between the central sleeve and the rolling elements, andextending in a generally radial direction and in facing relationshipwith the said annular transverse rear end face of the central sleeve,the said front ring having an annular extension extending radiallyoutwards beyond the ring and having a transverse front face in facingrelationship with the said transverse rear face of the radially innerportion of the piston;

[0017] the hub includes a radially oriented plate portion extendingoutwards, and in that [sic] the said axial thrust bearing is disposedfacing the transverse front face of the said plate portion;

[0018] the turbine wheel includes a generally radially oriented innerplate element or is connected to such a plate element, which is fixed tothe plate portion of the hub;

[0019] the plate element of the turbine wheel is adjacent to thetransverse front face of the plate portion of the hub;

[0020] the plate element of the turbine wheel is interposed axiallybetween the plate portion of the hub and the axial thrust bearing;

[0021] the axial thrust bearing includes a rear ring interposed axiallybetween the assembly consisting of the turbine wheel and hub;

[0022] the rear ring of the axial thrust bearing is centred radiallywith respect to the plate portion of the hub, or with respect to thesaid plate element of the turbine wheel;

[0023] the rear ring of the axial thrust bearing is centred radiallyoutwardly with respect to the inner periphery of the said plate element;

[0024] the rear ring of the axial thrust bearing is centred radiallyinwardly with respect to the plate portion of the hub;

[0025] the front ring of the axial thrust bearing is centred radiallywith respect to the central sleeve;

[0026] the front ring of the axial thrust bearing is centred radiallywith respect to the radially inner portion of the piston;

[0027] one ring of the axial thrust bearing is coupled in rotation tothe turbine wheel, or to the central sleeve, or to the piston;

[0028] the axial thrust bearing is attached axially to the turbinewheel, or to the central sleeve, or to the piston.

[0029] Further features and advantages of the invention will appear on areading of the following description, for an understanding of which,reference will be made to the attached drawings, in which:

[0030]FIG. 1 is a half view in axial cross section of a hydrokineticcoupling apparatus which includes an axial thrust bearing made andarranged in accordance with a first embodiment of the invention;

[0031]FIG. 2 is a scrap view on a larger scale, showing the lower righthand part of FIG. 1, in a second embodiment of the invention;

[0032] FIGS. 3 to 11 are views similar to that in FIG. 2, and show thethird to eleventh embodiments of the invention.

[0033] In the following description those components which areidentical, similar or analogous will be designated by the same referencesigns.

[0034] To facilitate understanding of the description and Claims, therewill be used, by way of non-limiting example, rear and frontorientations which correspond respectively to the left and right handsides of FIG. 1 and similar Figures.

[0035] As is known in the state of the art, a hydrokinetic couplingapparatus includes, arranged in a common sealed housing 10 filled withoil and constituting a casing, a torque converter 12 and a lock-upclutch 14.

[0036] The torque converter 12 comprises a turbine wheel 12, an impulsewheel 18 and a reaction wheel 20.

[0037] The impulse wheel 18 has blades 22 which are carried by a firsthalf shell 24, which is sealingly secured to a driving second half shell26 arranged to be coupled in rotation to a driving shaft.

[0038] The turbine wheel 16 also has blades 28 which face towards theblades 22 of the impulse wheel 18, and the turbine wheel 16 is coupledin rotation to a hub 30 which is arranged to be coupled in rotation to adriven shaft (not shown) coaxial with the axis X-X of the apparatus, bymeans of splines 32 formed on the inner periphery of the radially innerportion 34 of the hub 30.

[0039] The driving half shell 26 consists essentially of a generallytransversely oriented wall 36, that is to say this wall lies in a radialplane at right angles to the axis X-X.

[0040] As can be seen in particular in FIG. 1, the turbine wheel 16 isfitted axially between the transverse driving wall 36 at the front andthe impulse wheel 22 at the rear.

[0041] The lock-up clutch 14 is fitted axially between the assembly thatconsists of the turbine wheel 16 with its hub 30, on the one hand, andthe transverse front driving wall 36 on the other hand.

[0042] The two half shells, the rear one 24 and front one 26, constitutethe sealed casing or housing 10 which, by means of its driving fronthalf shell 26, constitutes the input element of the hydrokineticcoupling apparatus, while the hub 30 of the turbine wheel 60 is theoutput element of the apparatus.

[0043] In the case of application to a motor vehicle, the driving shaftconsists of the crankshaft of the internal combustion engine of thevehicle (not shown), while the driven shaft consists of the input shaftof the transmission of the vehicle (not shown).

[0044] The said driven shaft has a central hole to act as a channel forfeeding hydraulic fluid under pressure to a variable control chamber 38which is delimited by the components of, firstly, the lock-up clutch 14,which comprise an annular disc 40 coupled in rotation releasably to theassembly consisting of the turbine wheel 16 and its hub 30.

[0045] During starting of the vehicle, the turbine wheel 16 is driven inrotation by the impulse wheel 18 by the flow of the oil contained in thesealed casing 10 between the blades 28 of the turbine wheel 16 and theblades 22 of the impulse wheel 18.

[0046] The lock-up clutch 14 is then disengaged, and the torqueconverter 12 is in service or in operation.

[0047] In order to avoid a loss of power which is due in particular tosliding effects between the turbine wheel 16 and impulse wheel 18, theturbine wheel 16 is subsequently, after starting of the vehicle, coupledsolidly to the driving front half shell 26 through the lock-up clutch14, which is therefore engaged, the converter 12 being then out ofservice.

[0048] In particular, reference may be made to the contents of thedocument WO-A-94/07.058 to find further details relating to the knownstructure and operation of a hydrokinetic coupling apparatus of thiskind.

[0049] The driving front half shell 26 is a metal pressing, and itincludes at its radially outer periphery an axially oriented annularflange which extends towards the rear, and which in this example isreceived radially within a complementary axially oriented annular flange44 which is part of the rear half shell 24, for assembly and closing ofthe sealed housing or casing 10, for example by means of a weld seam 46.

[0050] As can be seen in particular in FIG. 1, the front transverse wall36 of the driving half shell 26 is continuous in its central part, thatis to say it has no machined aperture, nor any integral hole.

[0051] The construction of the driving front half shell 26 is thereforeparticularly simple, simply obtained by press-forming, and because ofits design it is generally sealed because it has no central hole orother peripheral hole, for example for carrying out fastening operationsby riveting internal components to the housing 10.

[0052] As can be seen in FIG. 1, the transverse front wall 36 carries,close to its periphery, elements 48 for coupling the half shell 26 inrotation to the crankshaft of the engine of the vehicle in a known way,by means of an interposed face plate not shown in the drawings.

[0053] The lock-up clutch 14 comprises a piston 50 which is a componenthaving the general form of a transversely oriented disc with a centralhole, and which is mounted for axial movement in both directions and insealed sliding movement on a central annular sleeve 25, called a guidesleeve, for guiding the piston, although guidance in axial displacementand in sealed sliding movement of the piston are ensured in this case bytongues.

[0054] For this purpose, the piston 50 includes a radially inner portion54 made in the form of an axially oriented annular tube element which ismounted for sliding movement, with a sealing ring 56 interposed, on acylindrical convex bearing surface constituted by the outer peripheralwall of the sleeve 52.

[0055] In the embodiments shown in succession in FIGS. 1 to 16, thedesign of the central sleeve is particularly simple, and is in the formof a monobloc cylindrical annular sleeve which is delimited radially byan outer peripheral surface 58 and a concave internal cylindricalsurface 60, and, axially, by two annular transverse end faces, namely afront face 62 and a rear face 64.

[0056] The centring sleeve 62, by virtue of its concave radially innersurface 60, guides the turbine wheel 60 in rotation with respect to thehub 30, with a sleeve bearing 66 interposed.

[0057] Thus, the piston 50, with the central sleeve 52 and thetransverse front end wall 36, delimits the variable volume controlchamber 38 which is delimited radially on the outside by the disc 40 andby the front friction disc 68 and rear friction disc 70, which areadapted to be gripped respectively between the disc or annular ring 40and the facing, radially outward, portion 72 of the transverse wall 36,on the one hand, and between the disc 40 and the facing, radially outer,portion 74 of the piston 50.

[0058] Each friction liner 68, 70 is coupled to one of the elements ofthe assembly 36, 40, 50, and the two liners 68 and 70 may for example becarried by the opposed transverse faces of the disc 40.

[0059] The disc 40 is the output element of the lock-up clutch 14, andin this example it extends radially outwards beyond the annularperipheral portion 76 of the piston, that is to say radially outside thepiston with reference to FIG. 1.

[0060] As in a known arrangement, the radially outer periphery 78 of thedisc 40 meshes with an axially oriented complementary portion 80 of amember 82 which is coupled in rotation to the turbine wheel 16, in thisexample by welding.

[0061] It is of course possible to provide an elastic damping devicedisposed at the periphery of the clutch 14, of the type comprising forexample circumferentially acting springs.

[0062] When the clutch 14 is engaged, the turbine wheel 16 is coupled tothe transverse wall 36 of the front half shell 26, so that either norelative movement takes place between the turbine wheel 16 and impulsewheel 18, or else, if sliding movement does occur, it is limited as afunction of the axial engagement of the clutch.

[0063] When the clutch 14 is engaged, the turbine wheel 16 is driven inrotation by the impulse wheel 18.

[0064] In the known way, the piston 50 is coupled in rotation to thetransverse wall 36, which turns and is fixed axially by means ofgenerally tangentially oriented resilient tongues 90, which are fixed toa driving or coupling member 92 so that the piston 50 is coupled inrotation to the transverse wall 36, while being displaceable axially inboth directions with respect to the latter by sliding axial movementalong the central sleeve 52, 58.

[0065] In the various examples shown in the drawings, the resilienttongues 90 are oriented tangentially, and they are riveted at one oftheir tangential ends to the piston 50, and at their other tangentialend, opposed to the first, to the coupling or driving member 92.

[0066] As can be seen in the drawings, the coupling member 92 isgenerally in the form of a generally radially or transversely orientedannular disc, and it has peripheral lugs 94 to which the correspondingtangential ends of the tongues 90 are fixed.

[0067] The tongues 90 and the coupling disc 92 are disposed axiallybetween the piston 90 and the front transverse wall 36, that is to saywithin the variable volume control chamber 38.

[0068] Because of the arrangement of the complementary portions 78 and80, and because they are meshed together, the disc 40 is coupled inrotation to the turbine wheel 60, but it is displaceable axially withrespect to the latter in such a way as to enable it to be grippedaxially under the action of the piston 50.

[0069] The radially outer lugs 94 of the coupling disc 92 lie in aradial plane which is offset axially towards the rear with respect tothe central portion 96 of the coupling member 92, this central portionbeing in the form of an annular ring portion which is interposed axiallybetween the central sleeve 52 and a facing internal annular surfaceportion 98 of the front transverse wall 36 which is oriented axiallytowards the rear.

[0070] The lugs 94 are connected to the central portion 96 by means of afrustoconical intermediate annular portion 100.

[0071] The central portion 96 of the coupling disc 92 is fixed to thetransverse wall 36 by welding, while the central sleeve 52 is alsowelded to the central portion 96 of the coupling disc 92.

[0072] With particular reference to FIG. 1, it can be seen that, inaccordance with the features of the invention, a needle bearing 84 isinterposed axially between the rear annular transverse end face 64 ofthe central sleeve 52 and the hub 30.

[0073] More precisely, the bearing 84 is interposed between the centralsleeve 52, 64 and a radial plate portion 86 which is an outward radialextension of the hub and on which the turbine wheel 16 is secured, bywelding in this case or, in another version, by riveting.

[0074] The front ring 108 of the needle bearing 84 is extended radiallyoutwards by a ring 88 which acts as an axial rear-facing thrust elementfor the piston 50.

[0075] More precisely, the transverse plate portion 86 of the hubextends radially outwards and is delimited axially at the front by afront transverse annular face 102, against which an internal radialplate element 104, connecting the turbine wheel 16 to the hub 30, bearsaxially at the rear, the plate element 104 being, in this non-limitingexample, made in the form of a radially inner portion of the shell ofthe turbine wheel 16.

[0076] The inner radial plate element 104 is fixed to the plate portion86 by welding, but it can of course be fixed in any other way, forexample by adhesive bonding or riveting in a known way.

[0077] The needle bearing 84 is a thrust bearing in which the needleswhich are the rolling elements 106 are disposed radially with respect tothe axis X-X, and are located between two rings, namely the front ring108 and rear ring 110, each of which is mainly in the form of a radialplate portion with an axially oriented annular portion, which is on theradially inner side of the front ring 108 and the radially outer side onthe rear ring 110.

[0078] The axial thrust needle bearing 84 is therefore a component whichis interposed axially between the rear transverse annular face 64 of thecentral sleeve 52 and a facing portion of the transverse front face 102of the plate portion 86 of the hub 30.

[0079] In the first embodiment shown in FIG. 1, the needle thrustbearing 84 is centred radially within the radially inner peripheral edge112 of the plate element 104 of the turbine wheel.

[0080] The needle bearing 84 is preferably mounted by means of its rearring 110, which is lightly gripped within the plate element 104, so thatit is coupled in rotation, and attached axially to, the assemblyconsisting of the turbine wheel 16, 104 and the hub 30, 86.

[0081] However, without departing from the scope of the invention, it isnot essential that the thrust bearing be coupled in rotation and/orattached axially.

[0082] As to the thrust ring 88, this is a radially outward extension ofthe plate portion of the front ring 108 of the bearing 84, extendingradially over a distance such that the ring 88 is situated, through itsfront transverse annular face 89, in facing relationship with the facingportion of the transverse rear face 51 of the piston 50, which isadjacent to its radially inner guide tube element 54.

[0083] Because it is in the form of an extension of the front ring 108,the ring 88, acting as an abutment for the piston 50, is also slightlyelastic so as to absorb shocks when the piston 50 makes contact with thethrust bearing.

[0084] The second embodiment shown in FIG. 2 is identical to the firstembodiment, except for the means coupling the needle bearing 84 inrotation with the hub 30 and the radial plate element 104, which areprovided by complementary teeth formed in the radially inner edge 112 ofthe plate element 104 and in the axially oriented radially outer portion114 of the rear ring 110 of the thrust bearing 84.

[0085] In the third embodiment shown in FIG. 3, it can be seen that theplate element 104 is extended radially inwards along the whole of theannular front transverse face 102 of the plate portion 86 of the hub 30,and that it is extended by a radially inner, axially oriented tubeelement 116 extending towards the rear.

[0086] The needle thrust bearing 84, the design of which is generallysimilar to that of the thrust bearing used in the first and secondembodiments, is thus interposed axially between the central sleeve 82and the plate portion 86 of the hub 30, with the radially inner plateportion 104 of the turbine wheel 16 being interposed.

[0087] For radial centring of the needle thrust bearing 84, in this caseit is the plate portion of its rear ring 110 that includes thetransverse, radially inward, extension 118 which is centred on the outerperipheral surface 120 of the tube element 116, and fitting may or maynot involve a slight gripping, so as to couple the rear ring 110 inrotation with the hub 30.

[0088] The fourth embodiment shown in FIG. 4 is a variant of the thirdembodiment, in which the rear ring 110 is driven in rotation by means ofmeshing lugs formed in the extension 118 of the plate portion of therear ring 110 with complementary notches 120 formed in the tube element116 .

[0089] In the fifth embodiment shown in FIG. 5, the needle thrustbearing 84 is carried by the central sleeve 52, the annular reartransverse end face 64 of which includes for this purpose a seatingbounded by a transverse face 122 and a convex cylindrical surface 124,the seating thus formed being open axially towards the rear to enablethe thrust bearing 84 to be fitted, the front ring 108 of the latterbeing engaged within the seating, with, if need be, a slight radialgripping effect such as to couple it in rotation with the central sleeve52.

[0090] The sixth embodiment shown in FIG. 6 is a variant of the fifthembodiment, in which the front ring 108 of the needle bearing 84 isfitted elastically within the seating 122, 124 of the central sleeve 52,complementary profiles being formed for this purpose in the axiallyoriented radially inner portion of the front ring 108 and in the surface124.

[0091] The seventh embodiment shown in FIG. 7 is a variant of the sixthembodiment, in which the plate portion of the front ring 108 includes aradial inward extension 126 in the form of radially oriented lugs whichare received in notches formed in the sleeve 52 in alignment with thebearing surface 124.

[0092] Thus, the needle thrust bearing 84 is attached axially to thecentral sleeve 52 by elastic insertion, and is itself coupled inrotation by a meshing mechanism consisting of the lugs 126 and notches128.

[0093] In the eighth embodiment shown in FIG. 8, the needle thrustbearing 84 is centred radially with respect to the piston 50.

[0094] More precisely, the transverse plate portion of the front ring108 is extended axially towards the rear by a centring tube element 130which is received within the concave cylindrical surface 55 of the guidetube element 54 of the piston 50, and it is then extended radiallyoutwards by the thrust ring 88.

[0095] There is of course a slight radial clearance between the tubeelement 130 and the tube element 54, so as to leave the piston 50 freefor axial displacements.

[0096] In the ninth embodiment shown in FIG. 9, the needle thrustbearing 84 is centred radially with respect to the central sleeve 52, towhich it is also attached axially.

[0097] To this end, the plate portion of the front thrust ring 108includes, at its radially inner periphery, an annular centring tubeelement 132 which extends axially towards the rear and is centred on acomplementary concave cylindrical surface 134 formed at the innerperiphery and at the rear axial end of the central sleeve 52.

[0098] To couple the front ring 108 and the central sleeve 52 togetherin rotation, fitting of the tube element 132 on the surface 134 may bemade with a slight radial interference.

[0099] It is also possible to achieve this rotational coupling inaccordance with the tenth embodiment shown in FIG. 10, in which thecentring tube element 132 is made in the form of axially oriented lugsreceived in notches 134 of the central sleeve 52.

[0100] The eleventh embodiment shown in FIG. 11 is similar to the firstembodiment in FIG. 1, from which it differs only in the design of theplate element 104, which is not an extension of the shell of the turbinewheel 16, but which is a separate component attached and fixed to theshell of the turbine wheel, and extended radially outwards to mesh withthe disc 40 carrying the friction liners.

[0101] In accordance with the invention, radial centring means for theaxial thrust bearing 84 are arranged with respect to the plate element104 or with respect to the piston 50, 54, as illustrated in the variousembodiments, or, in another version, with respect to the hub 30, 86.

[0102] The invention is not limited to the embodiments just described.

[0103] In particular, in all cases it is possible to providecomplementary adhesive bonding of the front or rear ring of the needlethrust bearing 84 which is coupled in rotation to the adjacent elementto which it is attached.

[0104] It is also possible to make the needle thrust bearing 84 in theform of a component which has only one ring, and which has rollingelements which roll directly against a corresponding surface of the hub30 or central sleeve 52.

[0105] The extension which constitutes the abutment ring 88 may ofcourse, by way of a variant which is not shown, be made as an extensionof the rear ring 110 of the needle bearing 84.

[0106] The rolling bearing may also be made in the form of a ball orroller bearing.

1. A hydrokinetic coupling apparatus, especially for a motor vehicle, ofthe type comprising: a casing (10) having a generally transverselyoriented wall (36) adapted to be coupled in rotation to a driving shaft;a turbine wheel (16) including an internal plate element (104) andcoupled in rotation to a hub (30) which is adapted to be coupled inrotation to a driven shaft; a lock-up clutch (14) interposed between theturbine wheel (16) and the transverse wall (36), and comprising, inparticular, arranged axially between the turbine wheel (16) and thetransverse wall (36), a generally transversely oriented piston (50), theradially inner portion (54) of which is mounted for sliding movementalong a cylindrical outer surface (58) of a central sleeve, an annular,transverse rear end face (64) of which lies facing a front transverseface of the assembly consisting of the turbine wheel (16) and hub (30,86), and of the type including thrust bearing means (84) which areinterposed axially between the annular transverse rear end face (64) ofthe central sleeve and the said front transverse face (102) of theassembly consisting of the turbine wheel and hub, whereby to limitfriction between the said two elements (16-30, 52) that are rotatablewith respect to each other, a portion (88) of the thrust bearing beingdisposed facing a rear transverse face (51) of the radially innerportion (54) of the piston (50), so as to constitute an axial abutmentdefining the maximum rearward position of the piston (50) when theclutch is disengaged, characterised in that radial centring means areprovided for centring the axial abutment (84) with respect to the hub(30, 86) or with respect to the piston (50, 54) or with respect to theplate element (104).
 2. Apparatus according to the preceding claim,characterised in that the abutment means consist of an axial bearing(84) with rolling elements, which is interposed axially between theannular transverse rear end face (64) of the central sleeve (52) and thesaid transverse front face (102) of the assembly consisting of theturbine wheel and hub, and in that at least one ring (108) of the axialthrust bearing (84) includes a radially oriented annular portion (88)which lies facing the said transverse rear face (51) of the radiallyinner portion (54) of the piston.
 3. Apparatus according to either ofthe preceding claims, characterised in that the axial thrust bearing(84) has at least one front ring (108) interposed axially between thecentral sleeve (52) and the rolling elements (106), and extending in agenerally radial direction and in facing relationship with the saidannular transverse rear end face (64) of the central sleeve (52), thesaid front ring having an annular extension (88) extending radiallyoutwards beyond the ring (108) and having a transverse front face (89)in facing relationship with the said transverse rear face (51) of theradially inner portion (54) of the piston (50).
 4. Apparatus accordingto any one of the preceding claims, characterised in that the hub (30)includes a radially oriented plate portion (86) extending outwards, andin that the said axial thrust bearing (84) is disposed facing thetransverse front face (102) of the said plate portion (86).
 5. Apparatusaccording to the preceding claim, characterised in that the turbinewheel (16) includes a generally radially oriented inner plate element(104) or is connected to such a plate element, which is fixed to theplate portion (86) of the hub (30).
 6. Apparatus according to claim 5taken in combination with claim 4, characterised in that the said plateelement (104) of the turbine wheel (16) is adjacent to the transversefront face (102) of the plate portion (86) of the hub (30).
 7. Apparatusaccording to the preceding claim, characterised in that the said plateelement (104) of the turbine wheel (16) is interposed axially betweenthe plate portion (86) of the hub (30) and the axial thrust bearing(84).
 8. Apparatus according to any one of the preceding claims,characterised in that the axial thrust bearing (84) includes a rear ring(110) interposed axially between the assembly consisting of the turbinewheel (16) and hub (30, 86) and the rolling elements.
 9. Apparatusaccording to claim 1 taken in combination with claim 8 and one of claims4 to 7, characterised in that the rear ring (110) of the axial thrustbearing (84) is centred radially with respect to the plate portion (86)of the hub (30), or with respect to the said plate element (104 of theturbine wheel (16).
 10. Apparatus according to the preceding claim,characterised in that the rear ring (110) of the axial thrust bearing(84) is centred radially outwardly with respect to the inner periphery(112) of the said plate element (104).
 11. Apparatus according to claim10, characterised in that the rear ring (110) of the axial thrustbearing (84) is centred radially inwardly with respect to the plateportion (86) of the hub (30).
 12. Apparatus according to claim 1 takenin combination with claim 3, characterised in that the front ring (108)of the axial thrust bearing (84) is centred radially with respect to theradially inner portion (54) of the piston (50).
 13. Apparatus accordingto any one of the preceding claims, characterised in that one ring (110,108) of the axial thrust bearing is coupled in rotation to the turbinewheel (16, 104), or to the central sleeve, or to the piston (50). 14.Apparatus according to any one of the preceding claims, characterised inthat the axial thrust bearing (84) is attached axially to the turbinewheel (16, 104, 30), or to the central sleeve (52), or to the piston.